This invention concerns an improved gas foam flooding surfactant and gas foam-forming mixtures wherein the surfactants are enriched in olefin disulfonate. The surfactants are particularly useful in a surfactant-enhanced gas flooding process where a surfactant solution and a gas are used to form a gas foam within a subterranean reservoir that will displace and recover hydrocarbons from within the reservoir. The use of disulfonate-enriched olefin sulfonate surfactant compositions and foamable mixtures of those surfactants in gas flooding operations is also described.
A variety of techniques have been used to enhance the recovery of hydrocarbons from subterranean reservoirs in which the hydrocarbons no longer flow by natural forces. One such technique is water injection, or water flooding, to force hydrocarbons from the subterranean reservoir by flowing water through the formation. Another technique is the use of gas injection, which also functions to force hydrocarbons from the subterranean formation. Gas flooding for oil recovery is frequently used subsequent to water flooding. To enhance the effectiveness of gas flooding, a miscible gas may be used to swell and reduce the viscosity of oil present in the formation.
Due to the low viscosity of gas, it will finger or flow through the paths of least resistance, thus bypassing significant portions of the formation, and resulting in early breakthrough at the production well. Also, due to its low density, the injected gas tends to rise to the top of the formation and "override" portions of the formation. The mobility of the injected gas, combined with variations in reservoir permeability, often results in an irregular injection profile. All of these factors may result in lower hydrocarbon recovery.
The overall efficiency of a gas flooding process can be improved with the addition of a foaming agent or surfactant which is introduced directly into the reservoir by means of a water or brine vehicle prior to injection of the gas. The surfactant should have sufficient foaming ability and stability to satisfactorily reduce mobility of the gas, thereby reducing its tendency to channel through highly permeable fissures, cracks, or strata, and directing the gas toward previously unswept portions of the formation. The surfactant should also be chemically and thermally stable and soluble in the aqueous phase present under reservoir conditions. A process for improving the efficiency of a gas flooding process by injecting a surfactant to produce a gas foam in-situ is revealed in U.S. Pat. No. 2,866,507.
It is an object of this invention to provide an improved gas foam flooding surfactant by which lower residual oil saturation levels are achieved. Another object of this invention is to provide an improved ga foam flooding surfactant which achieves sweep efficiencies better than those which may be obtained through use of commercially available olefin sulfonate surfactants.